The Role of Anodising Parameters in the Performance of Bare and Coated Aerospace Anodic Oxide Films

Abstract

The anodising process parameters (voltage, temperature, and electrolyte) control the morphology and the chemical composition of the resulting anodic oxide film by altering the balance between oxide growth and oxide dissolution reactions. The porosity of the oxide film is reduced by the addition of tartaric acid to a sulfuric acid electrolyte, while anodising at elevated temperatures enhances oxide dissolution, leading to wider pores and rougher surfaces. No significant changes in the oxide chemical composition as a function of anodising parameters was found; in particular, no tartrate incorporation took place. The resistance of uncoated anodic oxide films against aggressive media and galvanic stress as a function of anodising parameters has been studied by electrochemical methods. Anodising in a mixed tartaric and sulfuric acid electrolyte improves the resistance of the anodic oxide against galvanic stress and aggressive media in comparison to sulfuric acid anodising processes. However, the corrosion protection performance of the anodic oxide films in combination with a corrosion-inhibitor loaded organic coating is not governed by the blank oxide properties but by the adhesion-enhancing morphological features formed during anodising at elevated temperatures at the oxide/coating interface.

An ORP-EIS approach to distinguish the contribution of the buried interface to the electrochemical behaviour of coated aluminium

“…The anodic oxide layer formed on aluminium substrates in the anodizing process provides an electrical insulator layer which is relatively inert. Therefore, with regard to the FE-SEM images, the anodic oxide layer in TSA-Substrate can improve the corrosion resistance of the underlying metal [30,32].…”

“…The sample studied in this work is a coated anodized aluminium for aerospace applications. A corrosion protection scheme in aerospace industry is typically a multilayer structure consisting of an anodized layer stacked by different layers of organic coatings [29][30][31]. In anodizing, an oxide layer is grown by anodically polarising the sample in an appropriate electrolyte which significantly improves the corrosion resistance of the substrate.…”

“…In anodizing, an oxide layer is grown by anodically polarising the sample in an appropriate electrolyte which significantly improves the corrosion resistance of the substrate. The oxide layer formed during this procedure in acidic solution consists of a continuous thin and dense underlying layer (barrier layer) covered with an outer thick, porous layer [30][31][32][33]. The coated anodized aluminium is a complex system where both coating and the anodic oxide film play a crucial role in the overall corrosion protection of the system and these two layers are integrated in each other at the interface due to the partial or complete diffusion of the organic coating in the porous section of the oxide [33].…”

Influence of the Anodizing Time on the Microstructure and Immersion Stability of Tartaric-Sulfuric Acid Anodized Aluminum Alloys

“…The structure of the oxide layer formed during anodizing, from the thickness to the pore diameter for example, depends on numerous parameters. The influence of these parameters (voltage, bath temperature, or nature of the electrolyte) on the anodic layer properties is well documented [5,[12][13][14][15][16][17][18][19]. For example, increasing the temperature of the bath increases the thickness of the oxide layer but can also impact the pore morphology [14].…”

The Effect of Ambient Ageing on the Corrosion Protective Properties of a Lithium-Based Conversion Layer

“…[5][6][7] Therefore, most aluminium alloys cannot be practically applied without adequate surface treatments. 8 Numerous surface treatment techniques have been developed to provide effective surface protection for aluminium alloys, such as organic coatings, 9,10 anodizing, [11][12][13] sol-gel synthesis [14][15][16] and chemical conversion films. [17][18][19] Chemical conversion coatings are often favoured because of their easy preparation process and costeffectiveness.…”